Transport Assessment and

Modelling for Development

Mike Salter

Transport Assessment Manager

Cambridgeshire County Council

Presentation Outline

• Why do we do Transport Assessments (‘TA’) ?

• What are the key steps ?

• Alternative approaches

• Why build and use models for TAs ?

• Real-world behaviour and models

• The CSRM and use for WING

• Summary

Why TAs ?

The TA Process

Alternative Approach 1

Smaller scale sites – ‘first principles’ - base trip generation estimates on surveys elsewhere

(‘TRICS’)

- assume new trip patterns based on observed local

movements?

- add these to existing, or growthed up, observations such

as traffic surveys

- test local impact with computer models of junctions,

against bus capacity, against existing pedestrian / cycle

networks…

- agree mitigation measures

Alternative Approach 2

Applicable to larger sites – transport modelling

What is a transport model ?

• “a mathematical representation of an area

of a transport system which can be used

to evaluate existing conditions and project

future effects brought about by changes

due to traffic growth and/or infrastructure

or land-use developments”

Why build and use models?

• Development impacts on transport system

• Strategy development

• Business case for transport schemes

• Comparison of scheme options

• Better understand likely operation

• Note: different scales of model – junctions, town-wide etc

Traditional approach…

Trip

Generation

Trip

Distribution

Modal split

Assignment

M25 (and 1990s A14?) forecasting approach

“fixed trip matrix”

Limited feedback

Limited representation of wider ‘real world’

behaviour

The simplified first principles approach to

development assessment is like this…

Real world responses to transport

• Change route

• Change mode

• Change time of journey

• Change destination

• Relocate…

CSRM

Study

Area

CSRM Transport Zones (300+ in total)

Transport

Networks

in CSRM

Version 3.0 TJG

23/03/09

Transport

Demand

Transport

Networks

Land Use

Local

Highway

Model

Employment,

Commercial

Developments and

Dwellings

Bus, Guided Bus,

Park and Ride,

Walk & Cycle

Schemes

Highway

Improvements

Inputs

Locations:

Employment by Sectors,

Household Types and

Population

Rental Indicators

Mode Choice:

Car, PT and Walk/Cycle

Demand

Travel costs and

Accessibility

Model Outputs

Journey Times,

Congestion,

Carbon Emissions

SATURN

Model outputs

15

Direct transport outputs

• Transport flows by mode and vehicle type

• Turning flows through junctions

• Boarding and alightings

• Travel times (and speeds) – links and junctions

Outputs to assess other impacts

Through comparison of ‘do-minimum’ and ‘do-something’

• Change in network performance - e.g. flows, congestion

• Environmental assessment – change in air quality, noise

• Other assessments – e.g. accessibility, social impacts

How do we know it works?

Covered by DfT “WebTAG” Guidance

– Also covers Transport Assessments

– HA also use WebTAG

WebTAG sets ‘validation standards’ - volumes and responses

CSRM tested to demonstrate:

– Fit with commuting and school trips

– Car flows crossing City boundary

– Bus passenger flows into City

– Park and Ride Site usage

– Boarding numbers at Rail Stations

– Cycling within Cambridge

Model responses are also checked:

– Response to changes in travel cost (decreased car mileage, mode shifts);

– Response to change in journey times, public transport fares

• But local checking and benchmarking are crucial, and close scrutiny of outputs.

In summary

• Clear guidance and best practice on how to do TAs – we

follow this in Cambridgeshire

• Real world responses to transport are varied – CSRM

captures these and can feed into TAs for larger more complex

sites such as WING

• Model is compliant with Government guidance

• A sophisticated tool for testing transport and development

proposals

• Still need to closely review inputs and outputs to ensure fit-for-

purpose

• But for smaller sites a more basic first principles approach is

more appropriate – eg: land north of Teversham Drift